One of the most important providers of balance is undoubtedly the spine. However, it is insufficient to consider the spine alone to examine the global sagittal balance. Dubousset et al. refers to the pelvis as the ‘pelvic vertebra’ to emphasize the functional integrity of the pelvis with the vertebral column.^[1]

Walking on two legs caused marked adaptations to the human locomotor system. The pelvis has become vertical, and the pelvis is opened to the sides to increase the active moment arm of the muscles attached to this region. The hips, with their strong muscle structure, have become the most important dynamic components of erect posture.^[2] Lordoses are formed in the vertebral column to increase the mechanical advantage against gravity. LL is one of the most important parameters of this adaptation and is a physiological phenomenon unique to humans.

Individual differences in LL create biomechanical advantages or disadvantages related to degenerative disorders that may occur. To understand these individual differences, sagittal balance and compensation mechanisms must be figured out. The two main variables in compensation mechanisms are PT and SS. The pelvic incidence (PI) concept, first defined by Duval-Beaupere et al., is equal to the sum of the two variables, PT and SS.^[3]PI is individual and does not change for life unless there is a fracture that will impair the sacropelvic mechanism.^[4] Thus, maintenance of sagittal balance occurs using the changes between PT and SS, but the sum of both will not change.

Recent studies have shown that the sagittal spinopelvic alignment has a critical importance in adult spinal deformity.^{[5, 6]} However, there is no standard sagittal balance in a normal population.^[7] The most economical posture around the gravitational force vector determines that person’s ideal sagittal balance. Some measurement methods have been developed considering the average values of the populations.^[7–9]

To ensure the harmony between the spine and pelvis, we need to know the sagittal spinal and spinopelvic parameters and their relationships. On lateral radiographs, cervical lordosis, TK, LL, SVA, PT, PI, and SS are the main sagittal parameters.

Muscle factors such as back extensor muscle strength and paraspinal muscle volume have been reported to be associated with decreased LL and spinal sagittal imbalance; however, it remains unclear what causes or accelerates spinal sagittal imbalance.^[10]

Sagittal imbalance causes increased muscle effort and energy expenditure, resulting in pain and disability. In the literature, an annual loss of muscle mass of approximately 1% has been demonstrated after the age of 40.^[11] As a result of muscle weakness, spinal balance deteriorates, and therefore elderly individuals have difficulties in supporting their weight in the lower extremities. The association of trunk muscle mass with low back pain has also been reported in elderly individuals.^[12] However, it is necessary to show the connection between spinal sagittal parameters and muscle mass in healthy individuals based on the initial point of origin.

The aim of this study was to provide normative values for spinopelvic parameters in adults and to investigate the relationship between PMTI and spinopelvic parameters in young adults using whole spine CT imaging.

Descriptive statistics of 113 consecutive cases (98 males, 15 females) are shown in Table 1. Distributions of all the parameters were normal except age and sex.

This study provided physiological values for pelvic and spinal parameters describing sagittal spinal alignment and PMTI in a cohort of 113 adults. The descriptive values of sagittal balance parameters were comparable to the studies of Hasegawa et al. and Huec et al., except for lower TK values.^{[8, 9]}

It was found that LL was correlated with SS and PI, and SVA was correlated with TK and PT. Also, PI−LL was determined to correlate with PI, LL, PT, SVA, and SS. Correlations between sagittal spinopelvic parameters were comparable with previous reports.^{[7, 8, 16]}

The ability of the human body to maintain its center of gravity with minimum energy expenditure is due to spinal sagittal balance.^[2] Spinal sagittal imbalance^[17], which causes low back pain and poor quality of life, is also one of the biggest concerns in the field of spine surgery. Insufficient compensation for spinopelvic alignment problems due to the decrease in LL has been indicated as the cause of spinal sagittal imbalance.^[18]

LL and the forces forming LL are very important to maintain balance in the sagittal plane.^[19] Among them, pelvic incidence is a critical parameter in changing the degree of LL.^{[20, 21]} Schwab et al.^[7] formulated the relationship between LL and PI with ‘LL=PI±9°’. A cut-off value of 10° for PI−LL mismatch and the predictive value of this cut-off in the success of lumbopelvic fixation operations were also shown.^{[7, 22]} In this current study, PI−LL is found to be related to PI. This result is supported by Hyun et al.^[23], who reported the ideal LL value in asymptomatic elderly, in contrast to the studies of Schwab et al.^[7] and Rothenfuh et al.^[22], who investigated the PI−LL value desired to be corrected in people with lumbar spinal pathology. Therefore, this formula needs to be supported by further studies for young adults, the elderly, asymptomatic people, and various pathological conditions.

The relationship between spinal sagittal imbalance and trunk muscle strength has been demonstrated in elderly people.^[24] However, the nature of this relationship in healthy young adults was obscure. Therefore, in this study the relationship between PMTI and spinopelvic parameters in healthy young adult individuals was also investigated. A person who has a sagittal balance disorder has to use compensatory mechanisms and contract the neck, back, waist, hip, and leg muscles more in order to get himself into a balanced state. Compensation mechanisms of the spinopelvic alignment problems are not innocent because they require a lot of energy, and the total load created by this energy is especially on the lumbosacral junction. Roussouly et al. used the crane model to describe the loads that the human spine is exposed to.^[25]

In order to maintain the balance in crane mechanics, the weight of the “basal contact point” in contact with the ground must be quite high. If the basal contact point of the human spine is acknowledged as the lumbosacral joint, gravity and contraction of the paraspinal muscles are assumed as the two main forces acting on this point. For example, with pelvic retroversion, the vector of gravity is displaced anteriorly.^[26] This change lengthens the moment arm of the gravity vector and increases the “basal contact load” acting on the basal contact point with the extended moment arm. To compensate for this, the contraction of the paraspinal and pelvic muscles will increase.

The psoas major muscle functions not only as a hip flexor but also as a stabilizer of the lumbar spine and thus plays an important role in maintaining ergonomic posture. PMT measurement was preferred because of its important dual task and easily applicable defined measurement method in imaging modalities.

In this study, an increase in PMTI, PT, and SVA and a decrease in SS were found in people with PI−LL >10. Ohyama et al. also investigated the relationship between sarcopenia and spinopelvic parameters in 126 individuals and reported greater SVA and TK in patients with spinopelvic mismatch in the sarcopenia group.^[27] Since mean TK was found to be lower in this study group than other population norms, no association could be found between spinopelvic mismatch and TK.

In the literature, a negative relationship between pelvic tilt and skeletal muscle mass in degenerative lumbar and cervical disease in advanced age has been reported.^[28] While SVA, TK, and PT tend to increase and LL tends to decrease with increasing age, the decrease in skeletal muscle mass is one of the important parameters affecting spinal misalignment.^[29] However, according to this study, psoas muscle activity increases as a compensation for spinal misalignment in young healthy adults. Therefore, clinicians should be aware that the compensation mechanisms in young and elderly individuals may be different and should consider these mechanisms in surgical and clinical decision-making.

This study has several limitations. Since the young adult trauma patients admitted to our hospital were mostly male, the gender distribution was not homogeneous. Because the study was a cross-sectional rather than a longitudinal analysis, temporal changes could not be investigated. PMT was the only parameter used in muscle mass analysis due to the retrospective nature of the study. Muscle strength testing and performance analysis should also be added to the measurement of muscle mass in future prospective clinical studies. The PMT/T1-S1 length (mm/m) formula used in the PMTI calculation was produced for this study because the height data of the cases could not be reached. Therefore, it cannot be compared with other studies. Since this study is a single-center retrospective study conducted in a tertiary university hospital, its generalizability is limited.

This study established normative values for spinopelvic parameters and PMTI in a healthy young adult population using whole spine CT imaging. Importantly, it revealed that the alterations in spinopelvic parameters and the associated increase in PMTI observed in cases of spinopelvic mismatch differ significantly from the patterns seen in elderly individuals and those with spinal pathologies. These findings highlight that compensatory muscle activity, particularly of the psoas major, plays a distinct role in maintaining sagittal alignment in younger populations, which may not directly parallel mechanisms in older adults. Understanding these age-related variations in spinopelvic alignment and muscular adaptation is critical for clinicians and spine surgeons in tailoring preventive, diagnostic, and therapeutic strategies.

None

There are no disclosures of the authors.

All procedures performed in studies involving human participants were in accordance with the ethical standards of the institutional and national research committee and with the 1964 Helsinki Declaration and its later amendments or comparable ethical standards.

Z.N.T. and B.D.K. designed the experiments, analyzed the data, and prepared the manuscript. Both authors approved the final manuscript.

The data that support the findings of this study are available on request from the corresponding author.